debian-repo-install-test: check if binaries report UNKNOWN as version string

In #3176 a new Jenkins job is created, that checks if the packages from the Debian repository can be installed on a plain Debian image that did not have Osmocom packages before. To make sure that all binaries can be started, we are calling them with --version in the test.

[...] in the past we also had the problem that some packages would generate UNKNOWN as version string. So as we're calling all binaries with --version anyway, it might be a good idea to also check that there's no UNKNOWN string present in the output?I guess the best would be to add the above new test as a new function, so the test output (ends up in jenkins console log) would clearly state what failed.

And indeed, as I was developing the test, I saw that many programs did output UNKNOWN as version string.